Chelate Effect on the Structure and Reactivity of Electron-Rich Palladium Complexes and Its Relevance to Catalysis

Moshe Portnoy, David Milstein*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

In order to clarify the origin of the “chelate effect” in catalysis by palladium, complexes of iPr2P(CH2)niPr2P (n = 2, dippe; n = 3, dippp; n = 4, dippb), Ph2P(CH2)3PPh2 (dppp), and PiPr2nBu were prepared and their structures, dynamic properties, and reactivities were compared. Pd(dippe)2 1d is a coordinatively saturated complex, both in solution and in the solid state. X-ray characterization exhibits a distorted tetrahedral geometry. The dippe bite angle is 87.05°. The compound crystallizes in the orthorhombic space group Pnna with a = 16.713(3) Å, b = 17.561(3) Å, c = 11.116(2) Å, V = 3277(1) Å3, Z = 4. Pd(dippp)2 (1a) and Pd(dippb)2 (1e) are coordinatively unsaturated, trigonal complexes and are in equilibrium with the binuclear complexes LPd(η2-L)PdL, 1b and 1f, respectively. Whereas 1d does not exhibit dynamic behavior, 1a and 1e undergo fast, intramolecular phosphine exchange, a process which is not observed with lb and 1f. The trigonal complexes (dippp)PdPiPr2Bu (1c) and (PiPr2nBu)3Pd were also prepared for comparison. The dippp complexes 1a–1c react with aryl chlorides to produce cis-(dippp)Pd(C6H4X)Cl as the major product and trans-(η1-dippp)2Pd(C6H4X)Cl as the minor one (X = 4-OMe, 4-Me, H, 3-OMe, 4-COMe, 4-CHO, 4-NO2). In contrast, the dippb complex le oxidatively adds chlorobenzene to yield only the trans complex (η1-dippb)2Pd(Ph)Cl. Reaction monitoring reveals that the cis and trans complexes are formed in parallel pathways. Cis/trans equilibrium is on the cis side for dippp and on the trans side for dippb. Reactivity toward chlorobenzene follows the trend Pd(dippp)2 > Pd(PiPr2nBu)3 ≫ Pd(dippe)2 ≫ Pd(dppp)2. These results are interpreted in terms of chelate stability, ligand basicity, concentration of the active 14e species and effect of the PPdP angle on its reactivity. The dippp ligand is unique in that it is the only one of those studied which results in Pd(0) complexes which (a) exhibit high reactivity in oxidative addition and (b) form cis complexes preferentially.

Original languageEnglish
Pages (from-to)1655-1664
Number of pages10
JournalOrganometallics
Volume12
Issue number5
DOIs
StatePublished - 1993
Externally publishedYes

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